PIS are investigators, police. They are not prosecutors, they bring the information they collect to a US Attorney- who works for the Attorney General in the Department of Justice, not the Post Office- who actually conducts the court cases.

In Law and Order: Postal Inspectors Jerry Orbach is a Postal Inspector, Sam Waterston is the Federal Attorney, and the closest boss who intersects in their respective lines of authority is literally the President of the United States.